At various times during the life of a well formed in a subterranean formation for the production of oil and gas, it is desirable to treat the well. Such treatments include perforating, gravel packing, fracturing, and acidizing. To perform these treatments, fluid loss control agents and diverting agents, also referred to as treatment agents in this specification, are used.
Although high fluid permeability is an important characteristic of a hydrocarbon-producing formation, high permeability will adversely affect the various treatments. During fracturing treatment, for example, it is desirable to control loss of the treating fluid to the formation so as to maintain a wedging effect and propagate the fracture. Therefore, the effective performance of some treatments of the wellbore require temporarily reducing permeability of formation strata to reduce the loss of treating fluid during the treatment. Several fluid loss control and diverting agents have been developed for use in these treatments, but many of them leave a residue in the wellbore or on the formation strata upon completion of the treatment. This residue can cause permanent damage to the production capability of the formation. Examples of materials used for reducing permeability include natural substances such as crushed limestone, rock salt, oyster shells, and silica flour. These relatively inert materials form a filter cake which may remain on the formation face and cause unwanted permanent pluggage. Other materials, such as salt, benzoic acid, and naphthalenes, have some water or oil solubility or may be sublimable. These materials are deficient in that it is often difficult to determine the well environment. Still other types of material include oil soluble, water insoluble materials such as soaps, gels, waxes, and resin polymers. These materials are intended to be removed by subterranean hydrocarbon fluids. However, contact with oil in microscopic rock pores is not guaranteed, thus, these materials may not be solubilized and permanent formation damage may occur.
The polyester polymers of hydroxy acetic acid with lactic acid are taught or claimed in U.S. Pat. Nos. 4,387,769 and 4,526,695, both being to Erbstoesser et al., as improved oil well treatment agents. These polymers were extensively studied in the 1950s as possible textile fibers until it was found that they readily hydrolized in the presence of heat and moisture. (See U.S. Pat. No. 2,676,945 to Higgins.) Erbstoesser et al. indicate that the polyester polymers are substantially insoluble in the wellbore fluid but degrade in the presence of water at elevated temperatures in about one to seven days to form oligomers which are at least partially soluble in the formation fluid and are readily removed from the well during production. These polyester polymers are expensive to manufacture and are of limited effectiveness in low temperature applications. A need still exists for a low cost fluid loss and diverting agent that has hydrolytic properties that permit use in oil wells having a broad range of ambient temperatures. This treatment agent must initially be insoluble in the wellbore fluid, must remain insoluble long enough to do the treatment, and should degrade quickly once the treatment is completed so that the well can be quickly returned to production. The treatment agent also should have a sufficiently high crystallinity and melting point to allow grinding to the desired particle size and to prevent melting or softening when being ground or used. Preferably, the treatment agent should be capable of being tailored to temperature conditions and desired timing of oil well treatment. That is, the time to degrade at a specific temperature should be adjustable. The present invention meets these requirements.
The composition of the present invention comprises an inexpensive, low molecular weight condensation product of hydroxy acetic acid with itself or compounds containing other hydroxy-, carboxylic-acid-or hydroxycarboxylic-acid moieties. The condensation products are friable solids with a melting point of greater than 150.degree. C. and generally with a degree of crystallinity. They have a number average molecular weight of 200 to 4000 and preferably are oligomers having a number average molecular weight of about 200 to about 650. They are primarily trimers up through decamers. They are insoluble in both aqueous and hydrocarbon media but will degrade at specific rates in the presence of moisture and temperatures above about 120.degree. F. to form soluble monomers and dimers. Rate of hydrolysis at a given temperature can be increased by incorporating small amounts of other molecules (usually less than 15% by weight) into the hydroxy acetic acid condensation reaction. These materials are usually flexible or more bulky molecules that partially disrupt crystallinity but leave the condensation product friable. Thus, the treatment agent can be tailored to adjust the rate of hydrolysis from a few hours to several days by controlling the amount and nature of the crystallinity.